Unit fuel injectors operated by cams, have long been used in compression ignition internal combustion engines for their accuracy and reliability. The unit injector, whether of the open or closed nozzle type, typically includes an injector body having injector orifices at one end and a cam driven injector plunger assembly mounted for reciprocal movement within a central bore formed the injector body. In a typical unit injector, fuel is metered into an injection chamber with the amount of fuel being controlled on a cycle by cycle basis. The plunger assembly reciprocates through an advancement stroke to pressurize the metered fuel and force the fuel through the injector orifices into an engine combustion chamber, and a retraction stroke to permit metering of injection and perhaps timing fluid for the next injection event.
One of the major challenges to unit injector designers is limiting injector plunger wear and failure. Injector plungers are sized to form a diametrical clearance between the outer surface of the plunger and the inner surface of the corresponding bore formed in the injector body. However, nonaxial forces acting on the injector body cause distortion of the injector body and thus the bore. As a result, the diametrical clearance between the plunger and bore is reduced causing undesirable plunger scuffing and wear, and possible seizure or failure of the plunger. One source of nonaxial or transverse forces on the injector body are threaded connections between an upper barrel of the injector body and a lower injector body assembly. Threaded connections inherently produce nonaxial or transverse forces in the injector body due to the angled surfaces of the threads.
For example, U.S. Pat. Nos. 4,467,772; 5,094,215; and 5,441,027 all disclose unit fuel injectors having an outer barrel threadably connected to a lower injector body assembly. The threaded connection is positioned axially along the injector directly radially adjacent a plunger bore. As a result, the threads inherently impart a transverse load to the injector body distorting the plunger bore and thus disadvantageously causing plunger wear and possibly failure.
Consequently, there is a need for a unit fuel injector having an improved connection between the injector body components which reduces nonaxial assembly forces thereby minimizing plunger wear.